The present invention relates in particular to a method of this kind in which, for at least two of the fields of view, the respective measurement is an angularly resolving measurement that is performed with different selections of several antennas used for transmission and/or for reception, a search for peaks in two-dimensional spectra of the baseband signals of the measurements of the respective fields of view being carried out in order to localize radar targets, and an angular position of a radar target localized in a respective field of view being determined on the basis of amplitudes and/or phases at the position of a peak, corresponding to the radar target, in two-dimensional spectra of the baseband signals which are obtained for the different selections of the antennas used for transmission and/or for reception.
The present invention further relates to a radar sensor, in particular for motor vehicles, that is configured to carry out this method.
In motor vehicles, FMCW radar sensors are used to detect the traffic environment, in particular to localize other vehicles.
The localization results can be used for a variety of assistance functions, for example for automatic separation control, automatic collision warning, or also automatic triggering of an emergency braking procedure in the case of an acute risk of collision.
In frequency modulated continuous wave (FMCW) radar sensors, a transmitted signal having a frequency modulated in ramp-shaped fashion is used. The signal is transmitted continuously during the course of the ramp. A baseband signal is generated from a received signal by mixing with the transmitted signal, and is sampled and evaluated.
The frequency of the baseband signal corresponds to the frequency difference between the signal transmitted at a given point in time and the signal received at the same point in time. Because of the frequency modulation of the transmitted signal, this frequency difference depends on the transit time of the signal from the radar sensor to the object and back, and thus on the distance of the object. Because of the Doppler effect, however, the frequency difference also contains a component that is conditioned by the relative velocity of the object. A measurement of the frequency difference on a single ramp therefore does not yet permit a determination of the distance and the relative velocity, but instead supplies only a linear relationship between those variables. This relationship can be depicted as a straight line on a distance-velocity diagram (d-v diagram).
There are conventional FMCW radar sensors that work with a sequence of identical, comparatively short ramps, called “rapid chirps,” which have a large frequency swing in relation to their duration and are therefore so steep that the distance-dependent component of the frequency shift dominates in the baseband signal while the Doppler shift is sampled by the sequence of ramps. A sufficiently high repetition rate of the short ramps is therefore necessary in order to arrive at an unambiguous determination of the relative velocity within a measurement region of the relative velocity. In particular, the time offset between successive short ramps must be less than half the period length of the Doppler frequency.
The radar sensor usually has several antennas that are disposed with a spacing from one another on a line, for example a horizontal line, so that different azimuth angles of the localized objects result in differences in the path lengths traveled by the radar signals from the object to the respective antenna. These path length differences result in corresponding differences in the phase of the signals that are received by the antennas and evaluated in the associated evaluation channels. The angle of incidence of the radar signal, and thus the azimuth angle of the localized object, can then be determined by equalizing the (complex) amplitudes received in the various channels with corresponding amplitudes in an antenna diagram.
In a multiple input/multiple output (MIMO) radar, a greater angular resolution capability is achieved by the fact that not only several receiving antennas but also several transmitting antennas are worked with, different combinations of transmitting and receiving antennas being evaluated and resulting in respective differences in the path length of a reflected signal.
In a MIMO radar, the signals transmitted with different selections of the transmitting antennas must be orthogonal to one another or separable in time. This can be achieved, for example, by code multiplexing, frequency multiplexing, or time multiplexing. The code multiplexing method requires a great deal of outlay, however, and enables only limited signal orthogonality. With the frequency multiplexing method the disadvantage exists that the phase and the Doppler shift are dependent on the respective wavelength. With the time multiplexing principle the problem exists that relative motions of the localized objects, in conjunction with the time difference between the measurements with different switching states, result in phase differences that complicate subsequent angle estimation.